Various standards are used in the industry for cable data transfer. For example, DOCSIS 3.1 is the new standard for Data-Over-Cable Service and the first to implement OFDM technology for cable upstream data transfer. Legacy Advanced Time Division Multiple Access (ATDMA) signals under DOCSIS 3.0 may remain deployed in the same cable plants for a very long time. The frequency band allowed for upstream is very limited. If a fixed bandwidth for Orthogonal Frequency-Division Multiple Access (OFDMA) is assigned in the early stages of deployment, such an OFDMA frequency band may be wasted if there are few DOCSIS 3.1 users. In the later stages of deployment, the ATDMA frequency band may be wasted if most of the users have moved to DOCSIS 3.1 modems.
In order to achieve better efficiency, ATDMA/OFDMA bands can be scheduled dynamically. However, field tests and simulations have shown that if the OFDMA fast Fourier transform (FFT) is performed on the combined signals, the ATDMA signal may have significant spectral spread on each side of it in the frequency domain due to a rectangular window function applied to OFDMA FFT. This can make a region on each side of the ATDMA signal unusable for OFDMA carriers. Some proposed approaches use extra band-pass filters to get the expected signal and narrow the guard band, but none of them change the mixer frequency to move the expected signal to the center of baseband dynamically.